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American Plutocrats

The probe to the edge of the solar system shows frontiers left to find.

July 14, 2015 7:40 p.m. ET

After travelling more than three billion miles through space over nine and a half years, a U.S. spacecraft buzzed by Pluto on Tuesday, gathering images and up-close observations of the last major astronomical body in the solar system left to explore for the first time. Thus the New Horizons mission is not merely testimony to human curiosity and ingenuity, but also the closing of a frontier.

The “Planet X” beyond Neptune was discovered less than a century ago by a Kansas astronomer, Clyde Tombaugh—some of his ashes are on board...